In a cellular communications system, a certain geographical region is divided into regions referred to as cells. The mobile stations (MSs) in each cell are served by a single base station (BS). A BS transmits information to a particular MS (or a group of MSs) in its cell on the radio path referred to as the downlink (DL), while the MSs transmit information to the BS on the radio path referred to as the uplink (UL). The transmissions on the UL and the DL may be on the same time intervals but on different frequency bands, referred to as frequency division duplexing (FDD), or on the same frequency band but during non-overlapping time intervals, referred to as time division duplexing (TDD). In some systems, the transmissions on the DL and UL are based on Orthogonal Frequency Division Multiplexing (OFDM) modulation. In OFDM modulation, the available bandwidth for the radio link (DL or UL) is divided into a large number of smaller-bandwidth units referred to as sub-carriers (SCs), onto which the information to be transmitted is embedded.
Due to OFDM modulation, on the UL, if the MSs in a cell simultaneously use non-overlapping SC sets to make transmissions to the BS, then when received at the BS, the transmission from any MS is rendered orthogonal to the transmission from any other MS. For example, MS(i) uses SC set {Si} to perform UL transmissions to the BS; and the SC sets used by different MSs are non-overlapping. Then, when received at the MS, the transmissions from MS(i) on SC set {Si} are not interfered with by any of the transmissions to the BS from any of the MSs j, where j≠i.
Similarly, on the DL, if the BS uses non-overlapping SCs to make simultaneous transmissions to different MSs, then at any MS, the transmissions meant for other MSs appear orthogonal to the transmissions meant for it. For example, the BS can transmit to MS(i) using SC set {Si}, and use non-overlapping SC sets to perform transmissions to various MSs. Then, when received at MS(i), the transmissions from the BS on SC set {Si} are not interfered with by any of the transmissions from the BS to any of the MSs j, where j≠i. This property of OFDM modulation allows simultaneous communications between several MSs and the BS on the UL, and the BS and several MSs on the DL.